The subject matter of this invention relates generally to undervoltage relays and more particularly to an undervoltage relay device in which both time-out and coil energization come from the same capacitive source.
Undervoltage relays for circuit breaker apparatus are well known. An example of such may be found in U.S. Pat. No. 3,590,325, issued Jan. 29, 1971 to J. W. McMillen et al. entitled "Undervoltage Protection and Energy Storage Trip Current". The latter patent is assigned to the assignee of the present invention. It is generally well known that the control systems for circuit breaker apparatus are often powered by the very electrical lines which they protect. It is feasible therefore to expect that under certain operating conditions, the electrical power necessary to trip the circuit breaker may not be present even though it is very desirous to trip the circuit breaker because of the amount of electrical current flowing through the contacts thereof. Such an occasion may occur when the voltage on the electrical lines from which the energization of the control system is derived falls below a certain minimal necessary level. To avoid this, it is well known in the prior art to purposely trip the circuit breaker when the control system voltage falls below the minimum operating value. The latter-mentioned patent shows an example of such a circuit where mechanical relays are utilized. It is desirous to find an undervoltage detecting and tripping system which utilizes static components because of the higher reliability, lower space requirements and power requirements associated therewith. Furthermore, it is desirous to provide a relatively short time delay period between the sensing of the undervoltage condition and the actuation of the tripping operation. This relatively short period, which may be as large as one half of a second, allows the conductors which are protected by the circuit breaker to continue to conduct current and thus serve electrical needs even though the voltage thereacross may momentarily drop below the level at which undervoltage tripping would otherwise be initiated. Consequently, inconsequential voltage transients and momentary voltage drops will be ignored although serious voltage drops of long duration will be accounted for. In the past, time delayed undervoltage devices have been found which utilize a two capacitive element system for operation. One of the capacitive elements stores the energy necessary to complete the energization of the coil once begun and the other capacitive element performs the timing function. It would be desirous if a time delayed undervoltage protective relay system could be found in which a single capacitive element could be used in place of dual capacitive elements. In the context used herein, the term "single capacitive element" does not necessarily mean that the system has only one capacitor. It means that whatever capacitive element or elements are used, they perform the multiple functions of timing out and providing energy for tripping. Consequently, two or three capacitive elements may be connected in series or parallel circuit relationship to form a single capacitive element. An example of the aforementioned dual capacitive element circuits (as opposed to single capacitive elements) is found in U.S. Pat. No. 3,343,036, issued Sept. 19, 1967 to F. L. Steen and entitled, "Static Undervoltage Circuit". Another example is found in U.S. Pat. No. 3,582,716, issued June 1, 1971 to R. O. Traina and entitled, "Apparatus for Providing Time Delay Upon Total Deenergization". In the latter patent, another disadvantage is found in that the signal for beginning initiation of the tripping operation occurs on an increase in voltage across the timing element (here a capacitor) rather than the decrease thereof. It would be therefore further advantageous if the present invention initiated the tripping operation on a decrease in voltage across the timing element rather than an increase in the voltage thereacross. It would be even further advantageous if apparatus could be found for providing a memory indication of a recent undervoltage trip where the undervoltage condition had disappeared after the tripping operation had been completed. It would be even further advantageous if apparatus could be found in which once the tripping operation had begun circuit variable levels (voltage for example) are changed to such an extent that a positive latching operation takes place.